The present invention relates to an image forming apparatus such as a laser printer or digital copier, and an optical beam scanning device available in such an image forming apparatus. Further, the invention relates to an optical component applied to an optical beam scanning device.
Conventionally, in order to suppress the variations in beam positions and defocus generation caused by the influence of temperature and humidity change or the like, there has been an optical beam scanning device with a correction optical element for producing opposite beam position change to the beam position change produced in a post-deflection optical system due to temperature and humidity change and generating opposite defocus to the amount of defocus generation provided in a pre-deflection optical system. For example, U.S. Pat. No. 6,337,757 discloses an optical beam scanning device having such a correction optical element.
In the case of such an optical beam scanning device, the device has been designed on the assumption that the temperature and humidity relationship between an optical element in a post-deflection optical system and the above described correction optical element in the pre-deflection optical system is a predetermined relationship. When the actual use conditions are different form the design conditions, sometimes the correction does not work properly, or contrary deteriorate the characteristics thereof.
For example, assuming that the correction optical element is designed under condition that the temperature and humidity are the same with the optical element in the post-deflection optical system and the correction optical element in the pre-deflection optical system and, when the temperature changes by ΔT, the amount of defocus generation in a sub-scanning direction in the post-deflection optical system is ΔDFCS (sub) and the amount of defocus generation in a main scanning direction is ΔDFCS (main), for canceling the amounts, the pre-deflection optical system is designed so that the amount of defocus generation in the sub-scanning direction is generated by—ΔDFCS (sub) and the amount of defocus generation in the main scanning direction is generated by —ΔDFCS (main) when the temperature change of the correction optical element in the pre-deflection optical system is ΔT (given that there is no temperature change in the post-deflection optical system). By the combination of them, when the temperature of the optical element of the post-deflection optical system and the correction optical element in the pre-deflection optical system changes by the same ΔT, the total defocus in the main scanning direction and the sub-scanning direction can be made zero. However, when there is no temperature change in the post-deflection optical system and only the temperature of the correction optical element in the pre-deflection optical system changes by ΔT, overall, the amount of defocus generation in the sub-scanning direction is generated by—ΔDFCS (sub) and the amount of defocus generation in the main scanning direction is generated by—ΔDFCS (main).
In practice, the post-deflection optical system and the correction optical element in the pre-deflection optical system are provided within an image forming apparatus as equal elements in the optical beam scanning device; however, in the image forming apparatus, heat sources such as a fixing unit for fixing toner on paper and cooling units such as a radiation fan are variously provided, and the conduction direction and convection of the heat from the heat sources and cooling units are not simple, and thereby, there is a possibility that the temperature changes are different between the post-deflection optical system and the correction optical element in the pre-deflection optical system.
Further, generally, the optical element of the post-deflection optical system is large under the necessity of passing a deflected beam, and the optical element of the pre-deflection optical system is located prior to scanning and small. Accordingly, the responsiveness to ambient temperature change is different, and there is a possibility that temperature changes are different between the post-deflection optical system and the optical element of the pre-deflection optical system.
When the size as a component is different, the complication of work for mounting with precision is also different. The mounting work takes a longer time for a smaller optical element, and even if it takes a long time to mount the element, the above described defocus problem occurs. In the case where mounting precision is not adjusted, when the distance between mounting reference planes is short, the influence by an error of the mounting planes or the mounting parts of the optical elements on the mounting angle becomes greater. The size is desirably larger for suppressing the inclination at the time of mounting of optical elements.